1. Field of the Invention
The present invention relates to a gene encoding glutathione synthetase from Candida utilis and Candida utilis in which intracellular content of glutathione is increased or decreased by means of the same gene, and food utilizing the same Candida utilis. γ-glutamylcysteine and cysteine produced therefrom, or glutathione and cystenylglycine produced therefrom are useful in food production.
2. Description of the Related Art
Cysteine is used for the purpose of improving food flavor, taste etc. While a proteolysis method, a semisynthesis method and so forth are known as methods for producing cysteine, the proteolysis method and the semisynthesis method are mainly used at present. In order to use cysteine for improvement of flavor and taste of food, natural food material having a high content of cysteine is being required. However, few natural food materials of such kind have been known heretofore. Meanwhile, it has been reported that a food material having a high content of cysteine can be obtained when yeast extract containing γ-glutamylcysteine is heated or treated with an enzyme (WO00/30474).
Cystenylglycine is a dipeptide in which cysteine and glycine are bound with each other through peptide bond. It has been reported that material for enhancing flavor of meat can be obtained by heating cystenylglycine together with sugar. Although it has been known that cystenylglycine is produced by peptide synthesis method, it has hardly ever been known that it is produced from natural materials. On the other hand, it is reported that food material having a high content of cystenylglycine can be obtained by heat-treatment or enzyme-treatment of yeast extract containing glutathione (JP2001-321117A).
In Saccharomyces cerevisiae, γ-glutamylcysteine is synthesized by the reaction of γ-glutamylcysteine synthetase using cysteine and glutamic acid as substrates. Further, glutathione is synthesized by the reaction of glutathione synthetase using γ-glutamylcysteine and glycine as substrates. Yeasts having a high content of γ-glutamylcysteine have been reported in WO00/30474; Otake et al., Agri. Biol. Chem., 54 (12): 3145-3150 (1990); Chris et al., Molecular Biology of the Cell., 8, 1699-1707 (1997); Inoue et al., Biochimica et Biophysica Acta, 1395, 315-320 (1998) and so forth. However, all of these reports relate to studies using Saccharomyces cerevisiae, and there has been no report about studies using Candida utilis. 
Candida utilis is an industrially important microorganism which can be used in production of biologically important substances including glutathione, some kind of amino acids and enzymes (Journal of Bacteriology, 1995, vol.177, No.24, p7171-7177). Candida utilis has characteristics that it takes most of its energy from the pentose phosphate cycle producing a pyridine base, and that it shows weaker catabolite repression, compared to Saccharomyces cerevisiae (Biotechnology, 3, 30 (1983)). Further, since Saccharomyces cerevisiae is usually used for research purpose, there have been few findings about Candida utilis. Under such circumstances, there has been no report about how Candida utilis synthesizes glutathione, and it has been only reported that a specific strain of Candida utilis obtained as a zinc resistant strain produced a large amount of glutathione at the temperature which is lower by 5° C. or more than the normal cultivation of yeast (JP03-18872B).
Thus, a gene encoding glutathione synthetase has not been reported in Candida utilis, and there have not been known methods of producing industrially-useful substances by controlling intracellular content of glutathione.